1. Technical Field
The present invention relates to bicycle frames that are engineered specifically for racing and high-performance, recreational riding. In particular, the invention relates to bicycle frames that are formed from lightweight and tough composite materials, and also to the methods by which such bicycle frames are produced.
2. Description of Related Art
Over the years, bicycle frames have been made of any number of different materials, primarily metals such as iron and steel, but more recently, in an attempt to reduce weight, aluminum, titanium, and composite materials have been utilized. A composite material is a heterogeneous material created from high-strength fiber reinforcements and an appropriate matrix material in order to maximize specific performance properties. More specifically, layers of graphite fibers, glass fibers, aramid fibers, polyethylene fibers or other fibers or combinations of unidirectional or woven fibers are formed as continuous, epoxy pre-impregnated layers that are commonly referred to as "prepreg."
In the case of frames formed of metal components, specifically, the frame has been formed of individual tubular components that are secured together by welding, brazing or the like. Of course, such techniques cannot be utilized in the case of frames formed of fiber and resin composite materials.
In the case of tubular frame components that have already been cured (the process by which the flexible prepreg is converted from a flexible material in a workable condition into a hardened structural condition), in order to form junctions between a pair of such components and a head tube insert, a bottom bracket insert or a seat tube insert, at the corners of the front triangle of a bicycle frame, in accordance with U.S. Pat. No. 4,900,048 to Darujinsky, the molded and cured components for forming the tubular portions of the frame are abutted with each other, an epoxy resin bonding material applied at the junction formed, and then one or more prepreg strips wrapped around the components to tie them together via an adhesive bond that is formed by curing of the prepreg strips. In the case of a frame that is assembled from uncured prepreg material, in accordance with various patents to Brent J. Trimble such as U.S. Pat. Nos. 4,850,607 and 4,902,458, prepreg materials for forming halves of the frame components are laid up in molds along with patches which define the junctions at which the head tube, bottom bracket and seat tube inserts are to be incorporated. The entire assemblage is cured and molded at the same time into at least the front triangle of the bicycle frame, the halves of the frame components and the patches all melding into a unified structure, as a result.
However, since the fiber reinforcement of such resin and fiber composite materials is capable of carrying a tensile stress loading that is many times greater than the resins with which they are impregnated, each seam represents a point of weakness. In order to enable transference of stress loadings from fiber to fiber across such seams, the edge areas of the pieces of prepreg must be overlapped in order to provide sufficient strength for transference of stresses. The strength of the composite in such overlapped regions (along the center line of two-part components and at the end of individual components of the frame) relies mostly on the adhesive interface in the overlap seam area. This, results in stress concentrations at the junctures, similar to the substantial stress concentrations between components of a conventional welded or brazed bicycle frame. Thus, to increase the strength of a bicycle frame made of composite materials, it is necessary to minimize or eliminate the presence of seams which represent stress concentration sites due to the relatively abrupt junctures between prepreg components or component sections, which can be as small as one inch in the case of the Trimble frames.